A quantitative radioimmunoassay for antibody directed against cell surface membrane antigens was developed. The method can be used also to measure antigen quantitatively. The cytotoxic DBA/2 killer cells that develop in the progressively growing P815 ascites have been characterized. They are Thy 1.2 positive and are non-H-2 restricted as they killed 4 out of 4 tested C57B1/6 tumor lines as well as 3 out of 5 DBA/2 lines tested. Studies have been initiated to characterize the membrane products controlled by the major histocompatibility complex of the rat, the AgB complex. The membrane products coded for by the K and D regions of the H-2 complex in the DBA/2 lymphoma L-5178Y have been analyzed. Preliminary data suggest that these glycoproteins differ in several respects from the molecules coded by the same regions in DBA/2 spleen; detergent sensitivity, lectin binding capacity and molecular size.